Add lua script

Added optional lua script as an alternative to running a shell or cmd script to refresh the images list.
Added .gitignore & .gitkeep for images folder

This script can be added to OBS (Tools > Scripts), and is run automatically when OBS is opened.
Slide mode & slide duration settings can now be updated using this script in OBS.

This script & browser source still need to be reloaded/refreshed when images or settings are updated. Reloading the (lua) script can be done from OBS (Tools > Scripts) Reload script button.